1. Field of the Invention
The present invention relates to a method and apparatus for reproducing digital or analog signals from a recording medium and, more particularly, for reproducing an analog signal from the medium when the corresponding digital signal cannot be reproduced.
2. Description of the Prior Art
The fidelity of the reproduction of an original analog signal that is magnetically recorded can be greatly enhanced if the original signal is recorded in digital form by a technique such as pulse code modulation (PCM). Then, when the digital signal is played back, it is re-converted into the original analog signal.
A problem encountered in digital recording and reproduction is erroneous digital information, that is, digital data that does not accurately represent the original analog signal. Techniques are available to "correct" all but the most serious data errors. Sometimes errors result from the absence of digital data being reproduced from the recording medium ("drop-outs"). In that event another approach is used: The original signal is also recorded on the medium in analog form and when drop-outs are encountered in the reproduced digital data, the analog form of the recorded signal is used as a substitute for the digital signal.
The implementation of that approach requires the detection of the drop-out and the timely substitution of the analog signal reproduced from an analog track on the recording medium. A conventional technique, disclosed in published Japanese Pat. No. 52-46806, published on Apr. 14, 1977, and applied for by TEAC on Oct. 13, 1975, in the name of Akira Sato, can substitute the analog signal for the digital signal in the event of a drop-out. A fault in that approach, however, lies in its inability to allow for drop-outs that are correctable and thus need not be replaced by the analog signal. It also does not substitute the analog signal when the uncorrectable errors in the digital data are caused by other kinds of data errors. Thus, the analog signal cannot be used as a substitute when other kinds of digital data errors are encountered.